Laminins are fundamental for development of many tissues. Mutations in laminins and related genes cause Congenital Muscular Dystrophies that include multiple peripheral nervous system abnormalities. Laminins and laminin receptors also affect cytoskeletal and RhoGTPAse molecules mutated in Charcot-Marie-Tooth neuropathies. Impaired laminin signaling causes arrest in the developmental step of radial sorting of axons by Schwann cells. Radial sorting is a prerequisite for myelination and a sophisticated example of how cell-cell and cell-matrix interactions cooperate to induce cellular polarization. In the two previous grant periods we showed that axonal sorting is a multistep process, and that the early steps of axonal recognition, segregation and wrapping require the laminin receptors ?61 and ?71 integrins and Rac1 RhoGTPase, whereas the subsequent detachment of large caliber axons by pro-myelinating Schwann cells requires the laminin receptor dystroglycan. We now propose to address three fundamental cell biological questions and roadblocks to our understanding of sorting and myelination: how do laminins and integrins, at the basal surface of Schwann cells, promote interaction with axons that occurs at the opposite surface (apical-like surface, near axons)? How are signals from laminins integrated with signals from axonal neuregulins? How are contact and wrapping of axons by myelinating glia initiated, mechanically powered and advanced? By capitalizing on the remarkable collection of mouse mutants and reagents that we have generated, we are now in the unique position to address these issues and to identify novel mediators in myelination. These studies will address the critical functional relationship between opposing polarized surfaces, which is relevant to the biology of any polarized cell, but is also important for radial sorting and the initiation of myelination; and is impaired in several human neuropathies and other myelin diseases.